A nasal visual field advantage in interocular competition

When our eyes are confronted with discrepant images (yielding incompatible retinal inputs) interocular competition (IOC) is instigated. During IOC, one image temporarily dominates perception, while the other is suppressed. Many factors affecting IOC have been extensively examined. One factor that received surprisingly little attention, however, is the stimulus’ visual hemifield (VHF) of origin. This is remarkable, as the VHF location of stimuli is known to affect visual performance in various contexts. Prompted by exploratory analyses, we examined five independent datasets of breaking continuous flash suppression experiments, to establish the VHF’s role in IOC. We found that targets presented in nasal VHF locations broke through suppression much faster than targets in temporal VHF locations. Furthermore, we found that the magnitude of this nasal advantage depended on how strongly the targets were suppressed: the nasal advantage was larger for the recessive eye than for the dominant eye, and was larger in observers with a greater dominance imbalance between the eyes. Our findings suggest that the nasal advantage reported here originates in processing stages where IOC is resolved. Finally, we propose that a nasal advantage in IOC serves an adaptive role in human vision, as it can aid perception of partially occluded objects.

elicit faster response times in both eyes. If the second was true we would expect that nasally presented targets only elicit faster response times in one eye but not the other.
Visual inspection of the data suggest that in both the dominant and recessive eyes the nasally presented targets elicited faster response times than temporally presented targets (see Supplementary Fig S2). Given that there were only 15 trials per eye (that is about 7 to 8 trials per target location, as location was randomized), it is remarkable that a statistically reliable effect was observed with only 36 observers, testifying to the strength of the RT difference between VHF. Visual inspection of the present data further suggests that the difference between RTs of nasally and temporally presented targets is most pronounced when the target is presented in the recessive eyes of the observers.

Responses
Instruction Report target's location (L/R of fixation) as soon as you can discern it.
Report target's location (L/R of fixation) as soon as you can discern it.
Report target's location (L/R of fixation) as soon as you can discern it.
Report target's location (L/R/U/D of fixation) as soon as you can discern it.

Recording
Keyboard presses: Left and right arrow, or A and D keys. Use index and middle finger of dominant hand. To confirm in one more experimental way whether the nasal advantage we have reported on related to interocular competition (IOC) specifically, we included an additional analysis. We could turn to a dataset which was retrieved from a student project supervised by author SG 1 .
In this breaking continuous flash suppression (b-CFS) experiment observers responded to targets which were presented to both eyes and to both VHFs, and, crucially for our cause, there  On the left, one trials of a binocular condition is shown, this is how a conventional b-CFS trial looks like: a target is gradually introduced to one eye, while the other eye is presented with a dynamic mask until the target is reported. On the right a monocular condition is depicted, where the only difference is that the target is gradually introduced to the same eye as the eye which is presented with the dynamic mask. As opposed to the binocular condition, in the monocular condition the target is not interocularly suppressed by the mask.
This dataset was standardized as described in the main manuscript, taking into account the presentation type (binocular or monocular) of the target stimulus. Specifically, this entailed that RTs were normalized by the presentation type as well as observer and eye of target presentation as in the other datasets.
As we were interested in the effect size of the nasal advantage in the two conditions we

performed a Bayesian RM ANOVA with factors 'presentation type' (binocular & monocular)
and 'VHF' (nasal & temporal). The BF in favor of the presence of an interaction was 4.33.

Subsequent directional Bayesian paired samples t-tests showed that in the binocular condition
it was 3.64 times more likely that the nasal targets elicited faster RTs than temporal targets, but in the monocular condition, it was only 0.731 times more likely that the nasal targets elicited faster RTs. Note that the stronger nasal advantage in the IOC condition (compared to the monocular condition) cannot be explained by a floor effect in the monocular condition diminishing any detection time difference, as the study's original effect-of-interest (described in ref. 1 ) was at least as (if not more) reliable in the monocular condition than in the IOC condition.
These results show that the advantage nasally presented targets have over temporally presented targets in response time, is present only when targets are interocularly suppressed. This finding suggests that origin of the nasal advantage we report on lies in the mechanisms resolving IOC.

Text S3: Two simultaneous targets Dataset
We interpreted the faster response times for targets presented in the nasal visual hemifield (VHF) compared to the temporal VHF as evidence that the nasal VHF has more competitive strength in interocular competition than the temporal VHF. Here, we consider the possibility that nasal and temporal targets were perceived equally fast (and thus had equal competitive strength), but that nasally presented targets somehow evoked faster response times.
To test whether nasally presented targets were actually perceived before temporally presented targets, we turned to one more breaking continuous flash suppression (b-CFS) experiment with an important adaptation in its design 1 . In this experiment, tw o targets are introduced simultaneously, one in each VHF (see Supplementary Fig. S4). Participants were instructed to report where (left or right) they saw a target appear first. With this adapted paradigm the outcome measure includes a response choice, reflecting which target was perceived first, regardless of response speed. Consequently, if the nasal advantage reflects a difference in detection times (rather than perception-unrelated response speed), we expect observers to report nasally presented targets to appear first more frequently than temporally presented targets. If the nasal advantage reported in the main manuscript was caused by a difference in response speed arising after interocular competition (IOC) is resolved, however, we would expect nasally and temporally presented targets to be reported to appear first equally often. Figure S4: Schem atic overview of b -C FS task w ith two targets Two targets (one left and one right of fixation) are gradually introduced to one eye, while high-contrast patterns are flashed (at ~10 Hz) to the other eye. As the patterns are much more salient than the targets, the targets will initially be suppressed from consciousness. Typically one of the targets will break though suppression (and become visible) sooner than the other. Observers have to report the location (left or right) of the target that appears first, as fast as possible. The reported location provides, besides a response time measure, a temporal order judgement measure: namely, which target broke through suppression first.
Nineteen observers successfully completed the experiment which consisted of 128 trials. For three observers a number (16, 5 and 9 respectively) of trials was excluded, as the response times were either too fast or too slow (i.e., outside the time window of 0.35 to 10 seconds). The eye to which the target was presented was randomized, but balanced within observers.
For each observer we counted how many of the first reported targets were presented in the nasal VHF. We then computed what proportion this was of the total number of trials of the observer. We found that on average observers reported to first perceive the nasal target in 66% (SD = 20%) of trials (see Supplementary Fig. S5). Naturally, in the other 34% trials the temporal target was reported to appear first. To test the hypothesis that the proportion of nasal-first reports were higher than chance (0.5) we performed a Bayesian directional one-sample t-test across participants. We found strong evidence (BF10 = 34.3) in favor of our hypothesis that nasal targets were more often perceived first. Figure S5: Proportion reported first perceived target locations.
The proportion of reported targets which happened to be in a nasal or temporal visual hemifield (VHF) for each observer (sorted). The bar on the left shows the average proportion across all observers. The dotted line represents the chance level, which was 0.5 as the two targets were always presented simultaneously. The error bar represents the bootstrapped 95% confidence interval.
These finding show that stimuli in presented in the nasal VHF are perceived earlier than targets presented in the temporal VHF, and thus indeed break though suppression faster. We want to stress that observers were presumably completely unaware of the fact that they are choosing nasal targets more often than temporal targets. From the observers' perspective, they were only indicating whether a target first appeared left or right of fixation. Because observers could not know -nor directly perceive -to which eye the targets were presented (and to which the mask), they could not distinguish temporally from nasally presented targets (e.g., left in left eye versus left in right eye). To conclude, we empirically demonstrated that the nasal advantage in b-CFS is based on a perceptual difference, and not merely based on a response bias.  However when we split the data into trials where the target was in the nasal visual hemifield (VHF) and trials where the target was in the temporal VHF, it is apparent that the difference in RTs between target locations is larger than the difference in high and low dominance of the faces (see Supplementary Fig. S7). This example shows important implications for researchers implementing the b-CFS paradigms. If for some reason the targets stimuli are not balanced over the nasal and temporal VHF locations -either by design or due to trial exclusion-the effect of VHF location can potentially skew the results. This could lead to not finding a sought after effect, or, arguably worse, falsely identifying a non-existent effect. Figure S7: R esponse tim es per V H F to stim uli classes w ith high and low dom inance rating . The same response time data as shown in Supplementary Fig. S6, but now the response times for targets within each condition are split over nasal and temporal VHF of target presentation. The effect of VHF of target presentation on RTs within a condition is more pronounced than that of facial dominance, which was one of the sought-after effects. Removing the large variance due to the VHF (a factor of non-interest) could be beneficial for statistical analyses of other factors of interest.
Our recommendation to account for this unwanted variance in the data is to normalize the response times over VHF. The benefits of normalizing data have been demonstrated before by Gayet and Stein 4 . In line with their method we propose the following extension of normalizing response time data. This is specifically useful for b-CFs experiments with lateralized targets.